The invention finds an advantageous, but non-restrictive, application in OFDM modulation communication, in particular in cognitive and frequency-agile communication.
The term “cognitive communication” is derived from the English term “cognitive radio” and designates intelligent communication in which the equipment is capable of changing frequencies dynamically and over a wide range of frequencies (e.g. several GHz, hence also the term “frequency-agile”).
Moreover, it is recalled that OFDM modulation (standing for “Orthogonal Frequency Division Multiplexing”) is used for splitting a high bitrate binary train into a multitude of low bitrate modulated trains (or channels). Each of these sub-channels is modulated by a different frequency, the spacing between frequencies remaining constant. These frequencies constitute an orthogonal base in which the OFDM signal spectrum has optimal occupancy in the allotted band. Thus, OFDM modulation distributes a high bitrate over a series of orthogonal low bitrate modulated subcarriers. These sub-carriers generally use narrow frequency bands.
One of the purposes of the present invention is preferentially to enable communication equipment to:                measure and characterize spectral occupancy over a range of frequencies in which this equipment is itself capable of operating, e.g. in an OFDM context, and        decide, according to the communication need and spectrum analysis, on the band in which to work.        
The present invention thus provides fine spectrum characterization of frequencies in particular for cognitive and frequency-agile communication equipment. It uses spectral scanning for this purpose. However, the present application uses the term “scanning” to mean the act of running through the spectrum, in a general way, whether for searching for a free band in the spectrum, or for detecting interference, etc.
Spectral scanning associated with a decision system has been proposed, particularly in documents such as WO-96/10300.
Digital spectral scanning is generally done with the aid of a Fourier transform, with, in particular, capture of the digital samples in a frequency band and a Fast Fourier Transform (or “FFT” hereafter). The result of the FFT corresponds to the signal spectrum in the band considered.
However, analysis by simple FFT does not faithfully represent the spectra present and therefore does not enable reliable identification and characterization of the corresponding signals. One of the possible reasons for this drawback would be associated with the rectangular window for time analysis. A pure sine wave, whose frequency does not exactly match one of the scanning FFT carrier frequencies, returns energy over a large number of carriers in a ratio with the energy of the strongest carrier which is neither controllable, nor predictable from one piece of equipment to another.
In addition, a conventional spectrum analyzer is extremely costly, chiefly for the following reasons:                it has to cover a very wide range of frequencies,        it has to cover variable resolutions,        finally it has the function of presenting the spectrum measured according to a large number of parameters.        
The present invention aims to improve the situation.